Method and apparatus for determining earth resistivities in the presence of extraneous earth currents

ABSTRACT

Method and apparatus for measuring apparent earth resistivity, by determining potential drop produced by electric current injected into the ground. Periodic, or very low frequency polarity reversal of direct current is used to eliminate induced voltage errors that might otherwise occur with higher frequency polarity reversal or alternating current. Synchronizing pulses are used to gather and separately store information from each half cycle. A calibrated readout is obtained in which values of the voltages in the two half-cycles are algebraically subtracted and divided by two to measure the potential drop, with automatic compensation for effects of earth currents.

This invention provides apparatus and method for measuring apparentearth resistivity. Such measurements are useful in various aspects ofexploration for mineral deposits, for example estimation of the depth tothe top of a clay or rock layer, determination of the sequence in whichlow- and high-resistivity layers exist and estimation of theirthicknesses.

There are several known methods and arrays for measuring apparent earthresistivity, such as the Wenner four-pin method, the Schlumberger, theLee five-pin method and others.

There are several commercially available earth resistivity instrumentswhich can be used to measure apparent earth resistivity values by theWenner four-pin method for example. As far as is known, none of thesethat would be considered hand held or portable has the capability ofproviding reliable measurements at pin spacings greater than 50, orperhaps at most, 100 feet. Again, as far as is known, these instrumentsare limited by available output power and the associated circuitry toobtain meaningful results. Furthermore, by reason of the relatively highfrequency of cycling the polarity of the square wave power outputsignal, these instruments are subject to introducing induced errors inmeasured voltages when using multiple take-out ground pin cable sets.

In one embodiment, the apparatus and method of the present inventionprovide means and method for obtaining satisfactory earth resistivitydata at pin spacings up to 400 feet. This enables determination of earthlayer sequence, thickness and resistivity from the earth's surface todepths of 200 to 300 feet. Such determinations are useful, for example,in prospecting for desirable depths at which to locate anodes forimpressed current cathodic protection systems.

The apparatus and method of the present invention also provide novel andimproved means and method for compensating for potentials produced byearth currents. The invention also minimizes polarization of ground pinsduring measurement and avoids induced voltage errors that mightotherwise occur in multiconductor cable sets.

According to the invention, direct current is employed with periodicpolarity reversals, the period preferably being between 0.25 and 4seconds, to minimize polarization of ground pins during measurement,thus eliminating the attendant errors, and to render unnecessaryprocedures such as the time consuming practice of taking a multiplicityof readings and averaging the results which would otherwise be requiredin order to deal with such errors. Automatic compensation is continuallymade for potentials produced by earth currents, naturally occurring orotherwise, which may be varying in both magnitude and direction. This isdone through memory-hold circuit, polarity reversing and differentialamplifier features. Direct current is used, instead of alternatingcurrent, in order to eliminate induced voltage errors that are likely tobe picked up in an alternating-current mode on voltage-measuring wiresin multiconductor cable sets that are run to ground pins. According tothis embodiment of the invention, multiconductor cable sets can be usedwithout induced voltage errors, while eliminating the separation orshielding of wires that would, it is believed, be required withalternating current supply.

The invention provides an automatic polarity reversing circuit withvariable cycle length control to minimize pin polarization, avoidinduced voltage error due to said polarization and optimize measurementefficiency. Preferably, a constant-current output circuit is used tohold the applied current to a desired value throughout the time, forexample two to ten seconds, during which the data for a particular pinspacing are being obtained. Values of current and voltage obtainedaccording to the invention are used to calculate a resistance by Ohm'slaw for the pin spacing being observed, and apparent resistivity iscalculated by appropriate known formula.

In one embodiment of the invention, current of about 10 to 100milliamperes, and preferably at least 40 milliamperes, is provided at upto 600 volts, depending on need; voltage above 600 is usuallyunnecessary. The use of such current and voltage levels makes itpossible to obtain satisfactory data at pin spacings up to 400 feet.Data can be gathered, checked and plotted to provide a resistivityprofile at such spacings, with sufficient points for adequatedefinition, in relatively short periods of time, for example one hour.

Other advantages of the invention will appear from the followingdescription of the invention.

The invention will be further described with reference to the drawing,which is a schematic diagram of the apparatus and electrical circuitryof one embodiment of the invention.

A cable/pin array 10 is comprised of pairs of multi-conductor cableswith multiple electrode takeouts and appropriate switching as known inthe art, to permit selection of any of the desired pin spacings aftersetting out a sufficient number of pins to accommodate several pinspacing values. For convenience in this embodiment of the invention andto produce output data plots with relatively uniform point separation onthe pin spacing axis of a log-log plot of pin spacing versus apparentresistivity, three pairs of cables are used sequentially. These cablepairs permit pin spacings of: (No. 1): 5, 10, 15, 20, 25, 35 and 45feet, (No. 2): 60, 80, 100, 130 and 160 feet and (No. 3): 200, 250, 300,350 and 400 feet. This arrangement provides a series of selectablecurrent and potential pin spacings for determining apparentresistivities and layer depths at pin spacings up to 400 feet. Whenswitched to a particular pin spacing, only those selected pin locationsare live, thus precluding short circuiting effects from other pins setout but not currently selected.

Storage battery 12 is typically of ten ampere-hour capacity for normaljobs. Conveniently, a vehicle battery can be used. Switching powersupply 14 converts 12 volt direct current obtained from battery 12, viaon-off switch and fuse 16, to either 300 volt direct current or 600 voltdirect current. The switching power supply 14 oscillates at 15kilohertz. Maximum current output is 100 milliamperes. Polarityswitching circuit 18 is comprised of dual double pole double throwrelays, dual relays being used to increase the insulationcharacteristics of the relay contacts. Polarity switching circuit 18,with dual relays, handles both current going to the current pins in thecable/pin array 10 and current returning from cable/pin array 10. Thelatter path continues through constant current control 19 and currentmeter 48 on its circuit completion return to high voltage power supply14. Current adjust potentiometer 20 establishes the desired current tobe injected into the earth. The desired current, as selected by currentadjust 20, is held stable for the entire measurement period by constantcurrent control 19 which has sufficient control capability to produceidentical output current for successive pin spacings, without furtheradjustment and read to tenths of a milliampere, for most commonlyencountered values of current pin (C1, C2)-to-earth contact resistancewithin a particular measurement site. Astable multivibrator 22 is asquare-wave oscillator whose frequency is adjusted by the cycle adjustpotentiometer 24, to between 0.25 and 4 seconds per cycle. The output ofthe circuit actuates relay driver transistors not shown and providesfiring pulses for monostable multivibrator 26. Dual monostablemultivibrator circuit 26 provides square-wave pulses to actuate currentinterrupt circuit 27 and provide synchronizing pulses to thesynchronizing signal circuit 28. Current interrupt circuit 27 interruptscurrent for 50 milliseconds at the beginning of each half cycle topermit relay switching with zero current flow. Synchronizing signalcircuit 28 provides square-wave synchronized pulses to actuatememory-hold amplifiers 36 and 38. Differential amplifier circuit 34amplifies the signal from the potential pins P1 and P2 and provides again of 2; the input is isolated from the output. Memory-hold amplifier36 stores in memory the output of differential amplifier 34 for onehalf-cycle. Memory-hold amplifier 38 stores in memory the output ofdifferential amplifier 34 for the alternate half-cycle. Differentialamplifier circuit 40 amplifies the difference between the output ofmemory-hold amplifier 36 and memory-hold amplifier 38; the amplifier 40has a gain of 1.25. Absolute value circuit 42 is an amplifier with again of 1, the output of which is always positive regardless of theinput polarity. Calibration potentiometer 44 scales the output of theabsolute value circuit 42. Digital voltmeter 46 uses an LCD readout todisplay potential P1 to P2, the output always being positive. Currentmeter 48 displays the current in milliamperes being injected into theearth. 12 volt direct current to 12 volt direct current isolated supply15 provides power to circuits 18, 19, 22, 26, 27 and 28 and providesisolation from the automobile battery. 12 volt direct current to 15 voltdirect current isolated supply 17 provides power to circuits 34, 36, 38,40 and 42 and also provides isolation from the automobile battery.Bucking circuit 30 provides a series potential to offset voltagesproduced by natural earth currents; it is used only with the auxiliarydigital voltmeter 32, which has an LCD readout to test circuitry and toserve as backup in case the primary measurement circuit fails.

The operation of the equipment shown in FIG. 1 is as follows:

Prior to start-up, the cable arrays 10 are placed on the earth'ssurface, with connections being made to steel rods or pins. Theresistivity determining apparatus is connected to storage battery 12.

The high voltage power supply 14 selector is placed in the 300 V.D.C.position. The voltmeter 46 power is turned on. The main power switch 16is then turned on.

The high voltage power supply 14 develops 300 V.D.C. and causes currentto flow through the earth via the dual reversing relay contacts 18 andelectrodes C1 and C2. The amount of current flowing is dependent uponthe setting of the current adjust potentiometer 20. If the desiredamount of current is not possible at 300 V.D.C., the high voltage powersupply 14 selector switch can be set at 600 V.D.C.

When the main power is turned on, the astable multivibrator 22 beginsoperating at a cycle rate dependent upon the setting of the cycle adjustpotentiometer 24. This rate is adjustable from 0.25 to 4 seconds/cycle.The output of the astable multivibrator 22 energizes or de-energizes thedual reversing relays 18, and provides starting pulses for the dualmonostable multivibrator 26 (one pulse each half cycle). The dualmonostable multivibrator 26 interrupts the current flow for 50milliseconds each half cycle at the time the dual reversing relay 18 isswitched to prevent high voltage arcing, and provides synchronizingpulses of 50 milliseconds duration to the synchronizing signal circuit28.

As the current is switched, (positively, then negatively, etc.) thevoltage developed across the two potential pins, P1 and P2 is in theform of a square-wave, with the potential dropping to zero for 50milliseconds each half cycle when the current is interrupted. Thispotential, usually in the millivolt range, is connected to buckingcircuit 30 and external digital voltmeter terminals 32, and to the inputof isolation amplifier 34 with a gain of 2. The output of isolationamplifier 34 is connected to memory-hold amplifier circuits 36 and 38.

The synchronizing pulses from the dual monostable multivibrator 26actuate two isolating photo transistors (not shown) in the synchronizingsignal circuit 28 (one each half cycle). This initiates sample and holdsignals for the memory-hold amplifiers 36 and 38. As a result, theanalog information for one half cycle is stored in memory-hold amplifier36, and the analog information for the remaining half cycle is stored inmemory-hold amplifier 38. For example, with a square-wave signal of ±10millivolts at the input of the differential isolation amplifier 34,whose gain is 2, memory-hold amplifier 36 stores (+)20 millivolts, andmemory-hold amplifier 38 stores (-)20 millivolts.) Each half cycle thememory-hold storage data is updated. The outputs of memory-holdamplifiers 36 and 38 connect to the input of differential amplifier 40whose gain is 1.25. Differential amplifier 40 amplifies the algebraicdifference of the outputs of the memory-hold amplifiers 36 and 38. Thus,the total gain of the readout circuitry is 5.0 (2 from differentialisolation amplifier 34, 2 from the algebraic subtraction of alternatepolarity cycles and 1.25 from differential amplifier 40.) The output ofdifferential amplifier 40 is connected to the input of absolute valuecircuit 42, whose output is positive regardless of the input polarity.Absolute value circuit 42 has a gain of 1. The output of absolute valuecircuit 42 connects to a calibration potentiometer 44, enablingcalibration. The calibration consists of setting the output voltage todigital voltmeter 46 to a value equal to the input voltage to isolatingdifferential amplifier 34 during one average half cycle. The algebraicdifference between the values in the respective half-cycles is thusdivided by two. For example, when the input voltage is (±)10 millivolts,the algebraic difference is 20 millivolts, and the output is calibratedto be + 10 millivolts).

Natural earth currents are always flowing. As a backup system, if thememory-hold circuit fails, the operation includes the use of a buckingcircuit 30 to cancel the potential developed by the flow of naturalearth current. This permits easier reading of voltages, which may drift,by making them more nearly the same on alternate cycles. The buckingcircuit serially adds a negative or positive potential to the measuringcircuit to initially establish alternate cycle voltages that are thesame. The bucking circuit and auxiliary digital voltmeter are only usedas backup or to verify the operation of the primary meter and itsmemory-hold feature.

For example, with a natural earth current flowing, which develops apotential of (+)5 Millivolts between P1 and P2, and with currentinjected into the earth to cause a potential drop of (±)10 millivolts,the input to the isolation differential amplifier 34 is (+)15 millivoltsfor one half cycle, then (-)5 millivolts for the succeeding half cycle.The output of isolation differential amplifier 34 with a gain of 2, is(+)30 millivolts for one half cycle, then (-)10 millivolts for thesucceeding half cycle. Memory-hold amplifier 36 stores (+)30 millivoltsand memory-hold amplifier 38 stores (-)10 millivolts. The output ofdifferential amplifier 40 is 40 millivolts×1.25 or 50 millivolts. Theoutput reading through calibration 44 and absolute value circuit 42 is(+) 10 M.V.)

Thus, although potentials developed by natural earth currents can behighly negative or positive, their effect is completely cancelled by theuse of the apparatus and method described.

According to the invention, the need to use a bucking circuit to producean easily read output is eliminated since the absolute value featuregives an output that is always positive and the memory-hold circuitmakes the output stable. A bucking circuit is unnecessary according tothe invention, although it may be used as a backup.

Preferably according to the invention, polarity switching is done duringa period of for example about 50 milliseconds when no current is flowingto prevent high-voltage arcing and consequent damage to relay contacts.Also, preferably, the polarity reversing cycle period is adjustable topermit optimization of measurement time and/or stability of voltageoutput readings. The current and voltage values appear as stablereadings and therefore do not have to be read simultaneously.Consequently, one person can operate the instrument and read current andvoltage sequentially. Preferably, isolation devices are used throughoutthe instrument to assure personal safety during high voltage operation.

The invention claimed is:
 1. Apparatus for measuring earth resistivitywhich comprises:(a) means for providing square-wave direct currentpulses, (b) means for injecting said current pulses through at least onepair of electrodes into the earth, (c) means for measuring the voltageof said pulses as received at electrodes in the earth at locationsspaced apart from the location of electrodes for the injection of saidcurrent, (d) means for amplifying said measured voltage received at theelectrodes, (e) two memory-hold amplifiers to receive said amplifiedpulses, (f) means for providing synchronizing of the pulses to saidmemory-hold amplifiers and for providing sample and hold signals to saidamplifiers, whereby analog information concerning said amplified voltageis stored for one half-cycle in one of said amplifiers and for theremaining half-cycle in the other of said amplifiers, (g) means foramplifying the difference between the output of the memory-holdamplifiers, (h) an absolute value circuit for providing positive outputregardless of the input polarity from the differential amplifier, and(i) voltmeter means for displaying potential between electrodes, 2.Apparatus according to claim 1 and additionally comprising means forswitching of polarity of electrodes in each half cycle between thepulses.
 3. Apparatus according to claim 1 wherein the frequency of saidpulses is between 0.25 and 4 seconds per cycle.
 4. Apparatus accordingto claim 1 and additionally comprising means for holding the current ofsaid pulses to a substantially constant value during the measurementwith a given electrode configuration.
 5. Apparatus according to claim 1wherein said pulses are provided at about 10 to 100 milliamperes and atvoltages up to about 600 volts.
 6. Apparatus according to claim 5wherein the current of said pulses is at least about 40 milliamperes. 7.Method for measuring earth resistivity which comprises:(a) providingsquare-wave direct current pulses, (b) injecting said pulses throughelectrodes into the earth, (c) measuring the voltage of said pulses asreceived at electrodes in the earth at locations spaced apart from thelocation of the injection of said current, (d) amplifying the voltage ofsaid pulses, (e) separately storing analog information concerning saidamplified voltage produced by each half-cycle of said pulses, (f)amplifying the difference in amplified voltage of each half-cycle andproviding the absolute value thereof; and (g) providing a calibratedreadout of the values of the voltages in the two half-cycles.
 8. Methodaccording to claim 7 wherein flow of said current into the earth isinterrupted for a portion of the pulse cycle during which the switchingof polarity of electrodes occurs in each half-cycle.
 9. Method accordingto claim 7 wherein the frequency of said pulses is between 0.25 and 4seconds per cycle.
 10. Method according to claim 7 wherein the currentof said pulses is held to a substantially constant value during themeasurement with a given electrode configuration.
 11. Method accordingto claim 7 wherein said pulses are provided at about 10 to 100milliamperes and at voltages up to about 600 volts.
 12. Method accordingto claim 7 wherein the current of said pulses is at least about 40milliamperes.
 13. Apparatus according to claim 1 which additionallycomprises a calibrating means, whereby the absolute value of theamplified differential voltage from the two half-cycles are divided bythe total gain of the readout circuitry to determine the potential dropproduced by said current pulses in passage through the earth.